It is planned to determine the three-dimensional structure of carbamoyl phosphate from Escherichia coli, a high energy intermediate used in several biosynthetic pathways. This enzyme catalyzes the formation of carbamoyl phosphate from glutamine, bicarbonate, and ATP, and is one of the first steps in the biosynthesis of pyrimidines. Conditions for flash-freezing of the crystal have been established and a search for heavy atom derivatives has been initiated.